Transcript Document

Electrostatics
http://www.bergwall.com/courses/
course/view.php?id=5
Assignment
Read Chapter 20 Static Electricity
Do these in Chapter 20 Static Electricity
20/10,13,47
I. Electric Charge
A. Neutral atom contains protons and
electrons in equal amounts
1. When atoms lose or gain electrons they
acquire a charge
a. Gaining electrons - acquire a negative charge
b. Losing electrons - acquire a positive charge
m= 10 -6
n = 10-9
p = 10 -12
Electric charge, cont.
B. By rubbing two objects together,
electrons can be transferred from one
object to another
1. like charges repel one another
2. unlike charges attract one another
3. charged objects attract uncharged or
neutral objects.
C. Field or force - region around a
charged object within which the force of
the charge can be detected.
1. Objects will move toward or away from the
charged object (depending on their charge) when
they enter this field of force
2. Strength of the force depends on
a. the larger the charge, the greater the force
b. the closer the object, the greater the force
(force follows the inverse square law - F a 1/r2)
Lines of Force
Lines of Force
• By convention they are drawn from
positive to negative.
Coulomb’s Law
F = k qaqb
d2
F, force in newtons, N
q, Coulomb - SI unit of charge.
1 C = charge of 6.24 x 1018 electrons or protons. Charge of 1 electron
or proton is the reciprocal of above # or 1.60 x 10-19 C
k = 9.0 x 109 N m2/C2
d, distance in m
Problem
Force of attraction
A
B
C
d=
d
Two equally charged pith balls, each of mass 0.10g, are suspended from the
same point by threads 20. cm long. The balls come to rest 10. cm apart due to
repulsion. Determine the charge q on each ball.
Draw out the situation.
Fg = mg = 1x10-4kg(9.8m/s2) = 9.8x10-4n
q
0.20m
Fr
0.05m
Fg
Find angle, q:
q= sin-1(o/h) = sin-1(.05/.2) = 14o
Fr = Fg tan q = 9.8x10-4 n(tan14o) =
Fr = 2.4 x 10-4 n
Fr = k qa qb / d2
q=
F r d2 / k
q=
2.4x10-4 n(0.05m)2 / 9.0 x 109 Nm2/c2
q = 8.2 x 10-5 c
D. Electroscope
• Any device used to detect an electric
charge
E. Charging Objects
1. Conduction - charged object transfer its
charge to a neutral object by touching
a. Result - same charge
2. Induction - placing a charge on an
object by placing it near a charged object
a. Result - opposite charge can be permanent if
object is connected to a much larger object
(ground)
Conduction
Induction
•online.cctt.org/.../lessonelectrostatics.asp
F. Conductor
• Any substance that permits electricity or
heat to move through it readily ; good
charge carriers
• Charges collect on outer surfaces and on
sharp or pointed surfaces of objects.
www.napa.ufl.edu/2002news/lightningrodph.htm
Insulators / Semiconductors
• Insulators do not conduct electricity well or
at all because they do not have free
electrons
• Semiconductors are between conductors
and insulators in their ability to transfer
charge
Cardinal Rules
Series:
E=Vt=V1+V2+V3
It=I1=I2=I3
Rt=R1+R2+R3
Ct=
1
1/C1+1/C2+1/C3
Parallel
E=Vt=V1=V2=V3
It=I1+I2+I3
Rt=
1
1/R1+1/R2+1/R3
Ct=C1+C2+C3
II.Static Electricity
• Accumulation of positive or negative
charges causing
– Lightning
– Shocks from door handles
– Clothes in dryer
Neutralized by
grounding objects
using fabric softener
D. Lightning
• Static electricity charges build up around 15,000 to 25,000 feet
above sea level
• When enough charge builds up, the charge moves downward
until it encounters something on the ground that is a good
connection.
• The return stroke is a flow of charge (current) which produces a
luminosity much brighter than the part that came down. This
entire event usually takes less than half a second.
Thunder is caused by lightning.
Extremely high temperatures (@ 50,000 oF) causes
rapid expansion of air which sends shock and
sound waves in all directions. What you hear is
the result of the expansion.
III. Uses of Static Electricity
• Van de Graaff generator
Robert Van de Graaff's
machine:
Friction at the bottom
deposits charges on a
moving belt
The belt carries charges up
to the interior of a metal
sphere
The charges move from
the belt and collect on the
sphere
Capacitors - a combination of conducting
plates separated by an insulator, used to store
electrical charge. Unit: farad or coulomb/volt
www.vk2zay.net/article.php/55
Variable Capacitor
• Capacitor with 2 plates that can move
different distances from one another
allowing a radio to pick up different
frequencies (tuning)
Capacitance
• C = q/V
• C, capacitance in farads
• q, charge in coulombs
• V, voltage (potential diff.) in volts
Capacitance Problem
• C = q/v
farad = coulomb/volt
• If a capacitor has a capacitance of 4 pf
with a voltage of 0.2 v, what is the charge
that is built up on the capacitor?
• q = Cv
• q = 4 x 10 -12 f (0.2v)
• q = 8 x 10 -11 c
• (p, pico, 10-12)
Capacitors in Series
• Math: reciprocally additive
1
Ct
=
1 + 1 …
C1 C2
Capacitors in Parallel
• Ct = C 1 + C 2 + …
Comparing Capacitors
• Placing different dielectrics (insulators)
between capacitor plates, produces
greater than capacitance than with just air.
• Another source:
• http://prettygoodphysics.wikispaces.com/P
GP+E%26amp%3BM
Capacitance info with building info
• “…this site … has a bunch of info about
historical leyden jars, how to build them
and how to make an electrostatic
generator…”
http://www.tufts.edu/as/wright_center/pers
onal_pages/bob_m/
• From:
[email protected] Mike
Maloney
Cardinal Rules
Series:
E=Vt=V1+V2+V3
It=I1=I2=I3
Rt=R1+R2+R3
Ct=
1
1/C1+1/C2+1/C3
Parallel
E=Vt=V1=V2=V3
It=I1+I2+I3
Rt=
1
1/R1+1/R2+1/R3
Ct=C1+C2+C3